Trump's political machine is gearing up to take out a top GOP senator who has criticized Trump

Jeff Flake, an Arizona Republican, has attracted the ire of the White House for his criticism of the president. Samantha Sais/Reuters Robert Mercer, a hedge fund billionaire and one of President Donald Trump's top benefactors, has donated $300,000 to a super PAC charged with defeating Arizona Republican Sen. Jeff Flake, an outspoken Trump critic, in the 2018 midterm election, Politico reported Wednesday.

A major Breitbart News funder who was brought into Trump's inner circle by White House chief strategist and Breitbart founder Steve Bannon, Mercer will fund former state Sen. Kelli Ward's Republican primary run against Flake.

The development is an ominous sign for Flake, whose approval rating among Arizonans recently dropped to 36%.

Flake refused to endorse Trump for president, skipped the Republican National Convention last summer, and called on the candidate to drop out of the race after the release of the "Access Hollywood" tape last October. At one point, Trump promised to spend $10 million out of his personal coffers to defeat the senator.

During a briefing last week, White House press secretary Sarah Huckabee Sanders said she didn't know whether Trump would personally finance the primary challenge, but she did not rule it out.

"Sen. Flake would serve his constituents much better if he was less focused on writing a book and attacking the president" and more focused on "passing legislation," Sanders said.

The White House has met with Ward and two other Republicans considering primary runs against Flake, according to Politico.